1. Field of the Invention
The present invention relates generally to ultra-wideband communication systems, and, in particular, to a receiver for use in an ultra-wideband communication system adapted to determine the location of an RF transmitter relative to an RF receiver.
2. Description of the Related Art
In general, in the descriptions that follow, we will italicize the first occurrence of each special term of art which should be familiar to those skilled in the art of ultra-wideband (“UWB”) communication systems. In addition, when we first introduce a term that we believe to be new or that we will use in a context that we believe to be new, we will bold the term and provide the definition that we intend to apply to that term. In addition, throughout this description, we will sometimes use the terms assert and negate when referring to the rendering of a signal, signal flag, status bit, or similar apparatus into its logically true or logically false state, respectively, and the term toggle to indicate the logical inversion of a signal from one logical state to the other. Alternatively, we may refer to the mutually exclusive boolean states as logic_0 and logic_1. Of course, as is well known, consistent system operation can be obtained by reversing the logic sense of all such signals, such that signals described herein as logically true become logically false and vice versa. Furthermore, it is of no relevance in such systems which specific voltage levels are selected to represent each of the logic states.
In the RF system topology shown in FIG. 1, it can be seen that, because of the non-zero angle of incidence, θ, the RF signal will arrive at one antenna before the other. In particular, it can be seen that the path to antenna A is greater than to antenna B by p=d*sin θ. In order to calculate θ, the angle of incidence, the time difference of arrival could be found. If d is relatively large then this would provide quite an accurate estimate of θ. On the other hand, if d is small the estimate turns out to be highly error prone.
FIG. 2 shows two receivers, 10a and 10b, which are clocked from the same crystal 12. If the same crystal 12 clocks identical phase locked loops (“PLLs”), 14a and 14b, the generated carriers that are supplied to the respective down converter mixers, 16ac-16as and 16bc-16as, will have the same phase. The RF signal will arrive at a slightly later time at antenna A than antenna B, so it will encounter a down converter carrier phase that is different in each of the mixers 16. If the baseband processors, 18a and 18b, are capable of calculating the complex impulse response of the channel, that impulse response will have a different in-phase (“I”) to quadrature (“Q”) ratio I/Q which is equal to the phase delay caused by the signal travelling the extra distance, p, before encountering the mixer 10a and being down-converted by the carrier. If the carrier frequency is high, e.g., 4 GHz or 6.5 GHz, then quite small distances, p, will lead to a relatively large carrier phase difference.sin θ=P/d  [Eq. 1]λ=c/f  [Eq. 2]
where:                f is the carrier frequency,        c is the speed of light, and        λ is the carrier wavelength.α/2π=P/λ  [Eq. 3]        
where:                α is the phase difference between the two carriers for the same point on the incident RF signal.p=αλ/2π (from Eq. 2 and Eq. 3)  [Eq. 4]sin θ=αλ/2πd (from Eq. 1 and Eq. 4)  [Eq. 5]θ=sin−1(αλ/2πd) (from Eq. 5)  [Eq. 6]        
Many angle-of-arrival (“AoA”) estimators require the use of multiple receivers, each listening to its own antenna, see, e.g., the First Related Application. In some AoA estimators, a single receiver is electronically switched between a plurality of antennas, see, e.g., the Second Related Application. In all such embodiments, the AoA approach tends to be problematic. In the Second Related Reference, several possible solutions are proposed.
Using two or more receiver antennas, the location of a transmitter can be found by using time of flight to get the distance to the transmitter and using the difference in either; time of arrival, or phase of arrival, to calculate an angle of arrival. However, we submit that it is unnecessary to calculate the angle of arrival in order to find that location. Further, we submit that what is needed is an improved method and apparatus for use in the receiver of a UWB communication system to determine the (x,y) location of a transmitter with respect to a receiver. In particular, we submit that such a method and apparatus should provide performance generally comparable to the best prior art techniques but more efficiently than known implementations of such prior art techniques.